Oxidative-reductive multi-stage bleaching of ground wood pulp



OXIDATIV E-REDUCTIV E MULTI-STAGE BLEACH- ING F GROUND WOOD PULP Mearl A. Kise, Robert W. Barton, and Robert H. Ingwall,

Portsmouth, Va., assignors to Virginia Smelting Company, West-Norfolk, Va., a corporation of Maine No Drawing. Application March 4, 1955 Serial No. 492,302

Claims. 01. 8-104) The present invention relates to the processing of mechanical or ground wood pulp and more particularly to a method of bleaching such material.

Mechanical or ground wood pulp is economically prepared by the simple disintegration of logs of wood against a rotating grindstone. The liberated fibers are constantly removed from the grinding apparatus by a flow of water therethrough. The resulting aqueous suspension is screened to remove therefrom particulate foreign bodies and fiber agglomerates and may thereafter be further refined. The pulp or slurry still contains lignins and resinous substances which, although partially compensating for shortness in fiber length by serving as a binder to increase the strength of the ultimate paper stock, impart a color and dullness to the paper stock.

Mechanical Wood pulp alone formsv the basis for much of the cardboard, wrapping papers and newsprint paper produced today. Mechanical wood pulp in combination with other fibers, chemical pulp, fillers, opacifiers and the like can be used in the manufacture of higher quality paper intendedfor printing purposes. Care must be taken, however, to maintain the mechanical pulp inclusions below the level at which their discoloration effects become objectionable.

Because of growing demands for quality paper, increased production costs, and material shortages, considerable attention has been directed toward refinements of the mechanical wood pulp in order that it could 'be employed to a greater extent by the industry; Conventional bleaching operations, involving calcium hypochlorite or sulfite solutions, have not been satisfactory. These require large quantities of bleaching solutions of high concentration, much of the effect of which is lost in side reactions, and are commercially impractical. vAdditionally, the eifects of the bleach are not always permanent and there is a subsequent partial reversion of color.

A more recent bleaching process involves the use of hydrogen peroxide or compounds capable of liberating active oxygen in solution. By careful control of operating conditions, ground wood pulp can be bleached satisfactorily. Treating costs remain high, however.

It is an object of the present invention to provide a composite bleaching process particularly adapted to the treatment of ground wood pulp wherein operating and material costs are lowered and at the same time superior bleaching and/ or brightness of the pulp results.

A further object of our invention is the provision of a composite bleaching process which enables the development within ground Wood pulp of a degree of permanent brightness surpassing that obtainable by other methods, taken singly'or in combination. 1

A still further object of the invention is to provide a composite bleaching process which will efiectively refine ground wood pulp and render it more acceptable as a component of high quality paper.

Other objects and advantages of our invention will be more readily apparent from the following detailed description of certain preferred embodiments thereof.

United States Patent Briefly stated, the present invention contemplates the hydrosulfite bleaching of ground wood pulp which has been pre-treated in a peroxide medium and stabilized with an acid sulfite or sulfur dioxide solution.

'Mechanical pulp of standard production has a fiber concentration within the range of 3 to 15 percent as determined by equipment limitations, and to expedite handling the concentration more generally falls within the range of 3 to 6 percent. While consistencies in this latter range are preferred for the hydrosulfite bleaching from a practical stand point, we are not necessarily limited thereto. Furthermore, it is quite possible for one phase of the process to be performed on a pulp of a concen tration differing from that of the remaining phases.

. In the pretreatment of the ground wood pulp, it is preferred to use sodium peroxide or a mixture of sodium and hydrogen peroxides although hydrogen peroxide,

I I sodium perborate or other alkali metal peroxygen compounds may be substituted therefor without deleterious eifect. The pretreatment requires from 0.5 to 3 percent by weight of the'oxidizing bleaching material, calculated on the weight of the dry wood pulp in the slurry being treated, a conventional method of expression. The pretreatment bleach is efiiected at a temperature of from 90 to 125 F. for a time (from one to eight hours) sufficient to exhaust about 98% of the oxidizing material contained therein. During the pretreatment bleach, the slurry is adjusted to an alkalinity of from 1.80 to 2 percent, and more specifically 1.88 percent calculated as NaOH, by proper balance of the bleaching materials being utilized With additions of sodium silicate and caustic or sulfuric acid where necessary. This addition of sodium silicate in an amount of about 5 percent (8.9% Na O, 29% 'SiO calculated on the weight of the dry wood pulp assists in the bleaching action as does a small amount, less than 0.1 percent, of magnesium sulfate. The latter components also provide a buffering and stabilizing action in the alkaline bleach solution.

The pretreatment bleach is accompanied by intermittent agitation of sufficient magnitude that all of the wood pulp is exposed to oxidizing media. The treating solution may conveniently be injected into the pulp slurry after screening and/ or thickening in transit to holding tanks. After the peroxide has been largely expended, the slurry is treated with acid sulfite or sulfur dioxide additions sufficient to reduce the pH of the aqueous suspension to within a range of from 6.0 to 7.0.

The second stage of our process involves a further bleach of this conditioned wood pulp with zinc or sodium hydrosulfite, or comparable 'deoxidizing agent, in an amount ranging from about 0.1 to 2 percent, again calculated on the amount of dry wood pulp present in the slurry undergoing treatment. The incorporation of not more than 1 percent by weight of a sequestering agent with the hydrosulfite solution assists in the proper functioning of the bleaching material although it is not essential thereto.

To insure maximum effectiveness of this reducing bleach medium, the temperature of the pulp slurry being treated should be maintained within the range of F. to F. We have observed that the reaction time is materially decreased as the reaction temperature is increased and accordingly production capacities can be substantially enhanced at the higher reaction temperatures.

Ordinarily, the second phase bleach of our process will be substantially completed within one hour. Some addij tional gains will follow longer exposure times, other conhydrosulfite, calculated on the amount of dry wood pulp in the slurry, is suflicient for effective bleaching thereof. A slight additional gain is experienced in the presence of larger amounts of bleaching agent and, accordingly, excesses over the' optimum amount specified may be employed and are limited only by the economies of the operation. It is of course to be expected that with increased amounts of bleach material there will be an accompanying increase in the length of reaction time required to expend the bleach media.

Because of the presence of iron in the wood pulp or process waters, we have found it desirable to include a small amount, from 0.1 to 1.0% by weight of a sequestering agent such as sodium tripolyphosphate or ethylene diamine tetraacetic acid, again calculated on the amount of dry wood pulp present in the slurry undergoing treatment, with the hydrosulfite to counteract this ferrous contaminant. For practical purposes 0.5 percent is suflicient and the slight additional gain possible with larger amounts of the tripolyphosphate in particular do not warrant the extra cost thereof.

To employ the reducing bleach effectively, a careful control of the pH of the treated slurry is necessary. Acid pulps will decompose the hydrosulfite before its bleaching action can be completed. It is, therefore, essential that pH of the slurry be adjusted to fall within the range of 8.5 to 5.0 before the reducing bleach is incorporated therein. Since the treatment with sulfur dioxide after the peroxide bleach lowers the pH of the slurry this operation provides a convenient means of control.

Of primary significance in the reducing bleach operation is the requirement for air free mixing and exposure conditions during the reaction period to avoid loss of bleaching media. Satisfactory additions of the treating solutions into the pulp slurry can be obtained with a stock pump. By equipping the pump with a screw feeder operating under a flooded feed, adding steam and phosphate solution near the feed end of the screw and the hydrosulfite at the intake of the stock pump, the entire mixing operation can be carried out readily. The mixed pulp slurry and reducing bleach media is then moved into largeholding tanks or chests. Suitable mechanical agitators may be used for additional mixing of the reaction mixture or a simple circulating pump system can be employed.

From the holding chests, after expiration of the reaction time, the pulp slurry is ready for utilization in the manufacture of paper without further treatment. Of course, fillers, extenders, opacifiers and the like can be blended with the slurry to meet special specifications.

Example In demonstrating the practical aspects of our process, a wood pulp composed of about 4 parts of spruce and 1 part of balsam fir was suspended in water at a pulp consistency of 11 percent. Aliquot portions thereof were treated with an oxidizing bleach medium containing 1, 2 and 3' percent by weight of sodium peroxide to which sufficient hydrogen peroxide had been added to obtain the necessary peroxide concentration and which contained approximately 5 percent by weight of sodium silicate (8.9% Na O+29.2% SiO and 0.07% of magnesium sulfate to insure a constant 1.88% alkalinity (calculated as NaOH) in the slurry. A small addition of NaOH was needed with the 1% concentration of Na O to obtain the specified 1.88% alkalinity. The peroxide bleaches were conducted at an average temperature of 110 F. until about 95% of the oxidizing material was expended, requiring from 1.25 to 1.75 hours. After bleaching, the

1 percent solution of either zinc or sodium hydrosulfite in the quantity necessary to obtain the desired reducing bleach concentration. Agitation and holding for completion of reaction under air-free conditions followed.

Handsheets were cast from the various samples, pressed and air dried. Brightness readings were made on these sheets, reading the sides which were exposed to the air during preparation.

Using a standard reflectance meter and standard reading procedures the following results were obtained.

Paper from unbleached pulp had a brightness reading of 56.6. Paper from the first stage oxidizing bleach had a brightness varying \from 61.6 to 68.7, depending upon the peroxide concentration to which the samples were exposed. These readings may be considered as the standards for comparision with the remaining readings.

The test paper produced from the pulp bleached for one hour at 115 F. using 0.75% sodium hydrosulfite and 0.25% of sodium tripolyphosphate following a 1% peroxide bleach showed a final brightness reading of 67.5. In other words, an additional 5.9 points of brightness resulted from the second stage bleach. The brightness of paper having a two hour treatment with a reducing bleach at 165 F. with 2% sodium hydrosulfite and 1% sodium tripolyphosphate following a 1% peroxide bleach was 72.1.

In this case the reducing bleach resulted in an additional 10.5 points of brightness increase. When the pulp, treated first with 3.0% peroxide, was bleached with 0.75 of sodium hydrosulfite and 0.25 of sodium tripolyphosphate for one hour at 115 F., the test paper produced had a brightness of 73.7, 5.0 points of which were contributed by the second stage bleach. By using 2.0%

slurry was diluted to 3 percent consistency and treated with gaseous sulfur dioxide until the pH thereof was reduced to 6.0.

Samples of the sulfur dioxide-treated slurry were reheated-and treated with sodium tripolyphosphate and a of sodium hydrosulfite and 1.0% of sodium tripolyphosphate for two hours at 165 F. following the 3.0% peroxide treatment, the brightness of the test paper Was 76.7 of which 8.0 points were from the second stage or hydrosulfite bleach. In these tests the residual peroxide was removed by treating the slurry with sulfur dioxide gas until a pH of 6 was reached.

Additional test paper readings demonstrate that approximately 1 point of brightness gain can be obtained for each 20 to 25 F. increase in temperature of reduction bleach. The gain is consistent regardless of the amount of oxidizing bleach entailed in the pretreatment of the paper stock.

The brightness readings on the test samples have indicated that variations in brightness gain caused by changes in the variables previously discussed are essentially the same regardless of the peroxide concentrations used in oxidizing bleach pretreatment. However, the amount of the brightness gain attributable to the second stage bleach is influenced to some extent by the amount of gain obtained with the pretreatment. The less extensive the pretreatment, the greater the gain with the second stage or reduction bleach. For example, the brightness gain due to bleaching with 1% sodium hydrosulfite and 0.5% sodium tripolyphosphate for one hour at 115 F. was 6.2 points with 1% peroxide as opposed to 5.7 points with the 3% peroxide pretreatment. The brightness gain With the sodium hydrosulfite alone was about 9.0 points. In other words, the more expensive peroxide treatment can be replaced to a marked extent by the hydrosulfite reducing bleach for lowest production costs.

Depending upon the nature and brightness of the raw stock, brightnesses of 75 to points (G. E. Meter), not previously obtainable with peroxide alone, can be obtained with the combination bleach of our invention. In addition, brightness gains of the same magnitude as those obtainable with peroxide bleaches can'be obtained with our combination bleach at greatly reduced operating and material costs. The brightness gains we have made possible are substantially permanent in the ultimate paper stock and the gradual discoloration noticed in paper prepared by other methods is not apparent herein.

It will at once be obvious that modifications and variations in the operating conditions of our proposed process are possible without departing from the spirit of the invention or the scope of the appended claims.

What we claim is:

1. A process of bleaching ground wood pulp in the form of an aqueous suspension containing approximately 11 percent by weight of pulp which comprises contacting at a temperature of from 90 to 125 F. said pulp with an oxidizing bleach medium containing from 1 to 3 percent by weight of sodium peroxide and about 5 percent by weight of sodium silicate, calculated on the dry weight of the wood pulp in said suspension, in alkaline solution for a time suflicient to expend approximately 98 percent of the oxidizing material present in the solution, acidifying said suspension with sulfur dioxide to approximate neutrality and thereafter subjecting said pulp at a temperature of from 90 to 190 F. to the action of a reducing bleach medium containing about 1 percent by weight of sodium hydrosulfite and from 0.1 to 1 percent by weight of sodium tripolyphosphate, calculated on the dry weight of the wood pulp in said suspension, in a solution of a pH ranging from 8.5 to 5.0.

2. A process of bleaching ground wood pulp as defined in claim 1 in which the aqueous suspension is diluted to a concentration of from 3 to 5 percent by weight of wood pulp prior to treatment with the reducing bleach medium.

3. A process of bleaching ground Wood pulp in aqueous suspension which consists essentially in contacting at a temperature of about 110 F. said pulp in an 11 percent concentration with an oxidizing bleach medium containing about 1 percent by weight of sodium peroxide and about 5 percent by weight of sodium silicate, calculated on the dry weight of the wood pulp in said suspension, in a solution of approximately 2% alkalinity, cal culated as NaOH, for a time sufiicient to expend approximately 98 percent of the oxidizing material present in the solution, acidifying said suspension with sulfur dioxide to approximate neutrality and thereafter subjecting said pulp in a 3 to 5 percent concentration at a temperature of about F. to the action of a reducing bleach medium containing about 1 percent by weight of sodium hydrosulfite and about 0.5 percent by weight of sodium tripolyphosphate, calculated on the dry weight of the wood pulp in said suspension, in a solution of a pH ranging from 8.5 to 5.0 for a period of from 1 to 2 hours.

4. A process of bleaching ground wood pulp as defined in claim 3 in which the reducing bleach is effected under essentially air-free mixing and exposure conditions.

5. A process of bleaching ground wood pulp in slurry comprising contacting the pulp with an oxidizing-bleaching agent comprising 0.5% to 3.0% of sodium peroxide percentage being calculated on the dry weight of the pulp, secondly acidifying the pulp with sulphur dioxide to approximate neutrality and thirdly contacting said pulp with a reducing-bleaching agent comprising 0.1% to 2.0% of a metal hydrosulfite from the group consisting of zinc and sodium, the hydrosulfite likewise being calculated on the dry weight of the pulp, said contacting with said reducing-bleaching agent being under air-free conditions in a solution ranging from neutral to slightly acid.

References Cited in the file of this patent UNITED STATES PATENTS Re. 19,470 Hirschkind Feb. 19, 1935 1,855,332 Blodgett Apr. 26, 1932 2,187,016 Craig Ian. 16, 1940 2,707,144 Sparrow Apr. 26, 1955 2,707,145 Sparrow Apr. 26, 1955 FOREIGN PATENTS 143,123 Australia Aug. 29, 1951 OTHER REFERENCES Reichert: Paper Trace L, 1944, 118, Tappi, 89-96. 

1. A PROCESS OF BLEACHING GROUND WOOD PULP IN THE FORM OF AN AQUEOUS SUSPENSION CONTAINING APPROXIMATELY 11 PERCENT BY WEIGHT OF PULP WHICH COMPRISES CONTACTING AT A TEMEPRATURE OF FROM 90 TO 125*F. SAID PULP WITH AN OXIDIZING BLEACH MEDIUM CONTAINING FROM 1 TO 3 PERCENT BY WEIGHT OF SODIUM PEROXIDE AND ABOUT 5 PERCENT BY WEIGHT OF SODIUM SILICATE, CALCULATED ON THE DRY WEIGHT OF THE WOOD PULP IN SAID SUSPENSION, IN ALKALINE SOLUTION FOR A TIME SUFFICIENT TO EXPAND APPROXIMATELY 98 PERCENT OF THE OXIDIZING MATERIAL PRESENT IN THE SOLUTION, ACIDIFYING SAID SUSPENSION WITH SULFUR DIOXIDE TO APPROXIMATE NEUTRALITY AND THEREAFTER SUBJECTING SAID PULP AT A TEMPERATURE OF FROM 90* TO 190*F. TO THE ACTION OF A REDUCING BLEACH MEDIUM CONTAINING ABOUT 1 PERCENT BY WEIGHT OF SODIUM HYDROSULFITE AND FROM 0.1 TO 1 PERCENT BY WEIGHT OF SODIUM TRIPOLYPHOSPHATE, CALCULATED ON THE DRY WEIGHT OF THE WOOD PULP IN SAID SUSPENSION, IN A SOLUTION OF A PH RANGING FROM 8.5 TO 5.0. 